bcachefs: bch2_dir_emit() - fix directory reads in the fuse driver

bcachefs: bch2_dir_emit() - fix directory reads in the fuse driver

Commit 0c0cbfdb84725e9933a24ecf47c61bdeeda06ba2 dropped the ctx->pos
update before the call to dir_emit. This breaks the userspace
implementation, causing the directory reads to be stuck in an infinite
loop. This doesn't happen in the kernel because the vfs handles the
updates to ctx->pos, but in the fuse implementation nobody updates
it.

Signed-off-by: Ariel Miculas <ariel.miculas@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>